1. Field of the Invention
The invention relates to an improved method of removing material from semiconductor bodies by lapping. The improvement is particularly applicable to the processing of a semiconductor material for devices in which the lifetime of minority carriers is of importance; for example, light emitting diodes, laser diodes, and solar cells.
2. Description of the Prior Art
An essential step in the production of many semiconductor devices is lapping the wafer to the desired thickness and smoothness. In the context of this application, "lapping" will be generic to both thinning and polishing. Thinning, inter alia, reduces the size and weight of the wafer (of importance for space solar cell applications and facilitates mirror cleavage (of importance for laser diode fabrication). Polishing removes work damage and improves surface quality.
For thinning, a mechanical abrasive lapping agent such as diamond paste or a chemical-mechanical abrasive such as Syton (Trademark of Monsanto Company) is generally used in conjunction with a rotatable lapping turntable. The device side of a wafer is bonded to a planar mounting plate by applying a suitable adhesive such as wax to the mounting plate and then pressing the wafer against the plate and adhesive. The mounting plate is placed within a lapping fixture adapted to hold the other side of the wafer (i.e., the substrate) against the rotating turntable. The translational motion between the wafer and the abrasive on the turntable causes the substrate side of the wafer to be ground down to the desired thickness and planarity. The wafer is usually polished to repair surface damage caused by the abrasive. For polishing, a chemical etchant soaked lapping pad is frequently used. Alternatively, the thinning could also be done using an etchant soaked pad instead of an abrasive on the turntable.
Although these lapping procedures are important to device fabrication, they may have deleterious effects on the device, particularly the occurrence of areas of poor radiative efficiency called large dark spot (LDS) defects. LDS are a major factor in reducing the yield of light emitting devices, e.g., Al.sub.1 Ga.sub.1-X As-GaAs double heterostructure laser diodes. These LDS can also be nucleation sites for the growth of dark line defects (DLD) which significantly shorten the operating lifetime of such devices.
Photoluminescence studies have revealed that there are some LDS present in the as-grown wafer, but electroluminescence studies of completed devices have shown a proliferation of LDS following device processing, such as the lapping techniques described above. LDS result in regions of non-luminescent material because of significantly reduced minority carrier lifetime, and devices having one or more LDS within their active region do not operate optimally.
Inasmuch as LDS have been observed in p-n junction active heterostructure devices of direct bandgap materials, prevention of LDS would improve the quality and operating lifetime not only of lasers, but also LEDs, photodiodes, solar photovoltaic cells, phototransistors, photodetectors, etc., all of which depend for their efficient operation on good minority carrier lifetime and freedom from excessive non-radiative recombination of minority carriers such as occurs in the LDS defects.
Several theories have been proposed to explain the nature of the recombination process associated with LDS defects. Johnston, Applied Physics Letters, Vol. 28, (1976), p. 140, has suggested that LDS are the result of strain induced nonradiative heterointerface states, i.e., that nonradiative recombination is enhanced by the presence of a strain gradient. Another theory by C. H. Henry suggests that damage, such as microcracks, extending into the active region siphons off normal diffusion current into space-charge recombination causing excess junction current.
Johnston, Applied Physics Letters, Vol. 24, (1974), p. 494, suggested that LDS were produced by elastic strain fields associated with microscopic physical damage. R. L. Hartman and A. R. Hartman, Applied Physics Letters, Vol. 23, (1973), p. 147, proposed that strain fields can be responsible for shortening the operating lifetime of laser diodes. Accordingly, a modified bonding procedure was suggested which alleviated stress created by the different thermal expansion coefficients of the diode, bonding material and header. P. Petroff and R. L. Hartman, Journal of Applied Physics, Vol. 45, (1974), p. 3899, suggested that perturbations of the crystal lattice could be nucleation sites for the growth of DLD which shorten device lifetime in laser diodes. Physically damaged regions of a crystal indicated by LDS can be such nucleation sites, partly responsible for premature failure. Johnston in Applied Physics Letters, Vol. 24, supra, pointed out that LDS were caused by stresses during processing from contact of the wafer with hard surfaces. Avoidance of hard materials during lapping was suggested to alleviate the problem.
Although the prior art indicates that avoidance of hardness during processing will alleviate the LDS problem, typical means of doing so create additional problems. For example, both a soft lap or a soft mounting plate will cause loss of substrate planarity. A soft lap refers to the hardness of the lapping turntable or the lapping agent with respect to the wafer. It is known in the art that a soft lap causes less work damage than a hard lap, but causes rounding of the wafer. A mounting plate soft enough to alleviate the stresses causing LDS would also cause lack of planarity. Planarity requires that the wafer be mounted and held without distortion during the lapping procedures. Typical candidates for a mounting plate intended to have a quality intermediate between "soft" (such as pitch) and "hard" (such as steel) would be soft rubber, plastic, or Teflon (Trademark of Du Pont). These are neither soft enough to prevent LDS formation nor hard enough to preserve planarity so that such compromise candidates are doubly unacceptable.